1. Field of the Invention
The invention relates to a transmission mechanism of a progressive gear, especially a steering gear for a vehicle, comprising a seating for each of an input shaft and an output shaft.
2. Description of the Related Art
Progressive gears are widely used. One of the most important applications consists of steering gears of all kinds, but principally steering gears of controlled transport means of all kinds, for example steering gears of vehicles.
A gear is an apparatus for transmitting or converting movements. Of interest herein are gears which convert a rotary movement of an input shaft into a rotary movement of an output shaft. In the case of a steering gear of a vehicle, the input shaft is directly or indirectly coupled to a steering wheel whereas the output shaft is coupled to a suitable mechanism which brings about a change in the direction of the vehicle to be steered. In the case of rack-and-pinion steering, such mechanisms customarily comprise a pinion, which is coupled to the output shaft, and a toothed rack, in which the pinion engages. When the steering wheel is turned, the rack is moved and, by means of track rods, turns the wheels that are being steered. Other examples of those mechanisms include, inter alia, cam-and-roller steering arrangements and recirculating ball steering arrangements.
In contrast to gears which have a linear relationship between the input shaft and the output shaft, progressive gears have a non-linear relationship between the rotation of the input shaft and the rotation of the output shaft. Using the example of a vehicle, the effect of turning the steering wheel through a particular angle is accordingly dependent on the position of the steering wheel at that moment. Advantageously, that effect is small when the steering wheel is in the straight-ahead position and becomes larger the further the steering wheel has been turned. At high speeds this ensures precise steering around the straight-ahead position because even a relatively large movement of the steering wheel gives rise to just a small change in the direction of travel. On the other hand, during maneuvering, once the steering wheel has been turned to a large extent, just a small rotational movement of a progressive steering mechanism will give rise to a relatively large change in driving direction. This is desirable, in particular when parking, in order to be able to change efficiently from one wheel lock position to the other.
EP 0 915 003 B1 (Wandfluh) puts forward a vehicle steering device having a variable transmission ratio, comprising a total of four axes of rotation all intersecting at a single point. The disadvantage of that recirculating ball steering device lies in the fact that the levers are not located in a plane but at steep angles to one another. This technically awkward and unstable arrangement is sensitive to tolerances and requires a high outlay on the accuracy of the parts in order not to have too much play. This results in high assembly costs because the bearings have to be precisely adjusted at great expense. As a result of the steepness of the recirculating ball steering device, the steering sensitivity is reduced because the feel for the steering is lost through the balls of the radial steering. The relatively dead steering in the region of straight-ahead driving changes rather abruptly, on turning the steering wheel through about 75°, to highly progressive steering, as can be seen from FIG. 6 of the mentioned specification. From the same figure it can also be seen that the maximum possible turning angle of the steering wheel is limited to 105° on each side, which is a further disadvantage of that arrangement because the entire steering range has to be covered in just 210°.
The specification DE 195 19 588 A1 (Honda) puts forward a further steering gear having a progressive characteristic. The variability of the transmission ratio in that steering gear is based on the fact that although the output shaft of the steering gear extends parallel relative to its input shaft it is arranged offset therefrom (eccentrically). For coupling the input shaft to the output shaft an intermediate element is arranged between the two shafts which basically has the function of a slider crank (a prismatic joint combined with a revolute joint) and transmits the torque from the input shaft—or from an extension connected to the input shaft so that it rotates together therewith—to the output shaft. The effect of a steering gear of such a kind does indeed tend in the desired direction because the steering response increases in a harmoniously progressive manner with increased turning of the steering wheel; however, it would be desirable for that characteristic to be more pronounced. This cannot be achieved using the technical arrangement put forward in the mentioned specification because in achieving that low degree of progression the technical outlay required for ensuring play-free behaviour is already prodigious.
EP 0 915 003 B1 gives an extensive list containing further known proposals for progressive steering gears together with the various technical arrangements and their shortcomings.